Qilong Yong

Effect of Cryogenic Treatment on Properties of Cr8-Type Cold Work Die Steel

The effect of cryogenic treatment on the properties of Cr8-type cold work die steel was investigated. The results show that cryogenic treatment increases hardness by decreasing retained austenite, but the degree depends on the austenitizing temperature. When quenching at lower austenitizing temperature, the steel can obtain higher toughness by cryogenic treatment substituting conventional treatment process. Cryogenic time has little effect on cryogenic treatment. Conversely, cryogenic temperature has a great effect on cryogenic treatment and the effect of cryogenic treatment is more obvious with decreasing cryogenic temperature. In addition, deep cryogenic treatment improves the wear resistance by precipitating more homogeneous specific carbides.

Effect of Niobium on Isothermal Transformation of Austenite to Ferrite in HSLA Low-Carbon Steel

Using thermomechanical simulation experiment, the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation. The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism, wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation, whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs, wherein, the nucleation rate of ferrite in the initial stage of transformation is low, and in the second stage. the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete. Using carbon extraction replica TEM, niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite. The mechanism of the effect of niobium on the isothermal transformation was discussed.

Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.

Microstructure and Mechanical Properties of Martensitic Stainless Steel 6Cr15MoVn

Martensitic stainless steel containing Cr of 12% to 18% (mass percent) are common utilized in quenching and tempering processes for knife and cutlery steel. The properties obtained in these materials are significantly influenced by matrix composition after heat treatment, especially as Cr and C content. Comprehensive considered the hardness and corrosion resistance, a new type martensitic stainless steel 6Crl5MoV has been developed. The effect of heat treatment processes on microstructure and mechanical properties of 6Cr15MoV martensitic stainless steel is emphatically researched. Thermo-Calc software has been carried out to thermodynamic calculations OM, SEM and TEM have been carried out to microstructure observation; hardness and impact toughness test have been carried out to evaluate the mechanical properties. Results show that the equilibrium carbide in 6Cr15MoV steel is M23C6 carbide, and the M23C6 carbides finely distributed in annealed microstructure. 6Cr15MoV martensitic stainless steel has a wider quenching temperature range, the hardness value of steel 6Crl5MoV can reach to HRC 60. 8 to HRC 61.6 when quenched at 1060 to 1100 °C. Finely distributed carbides will exist in quenched microstructure, and effectively inhabit the growth of austenite grain. With the increasing of quenching temperature, the volume fraction of undissolved carbides will decrease. The excellent comprehensive mechanical properties can be obtained by quenched at 1060 to 1100 °C with tempered at 100 to 150 °C, and it is mainly due to the high carbon martensite and fine grain size. At these temperature ranges, the hardness will retain about HRC 59.2 to HRC 61.6 and the Charpy U-notch impact toughness will retain about 17.3 to 20 J. A lot of M23C6 carbides precipitated from martensite matrix, at the same time along the boundaries of martensite lathes which leading to the decrease of impact toughness when tempered at 500 to 540 °C. The M3C précipitants also existed in the martensite matrix of test steel after tempered at 500 °C, and the mean size of M3C precipitates is bigger than that of M23C6 precipitates.

Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process (TMCP), a high Nb low Mo fire resistant steel with the yield strength (YS) of 521 MPa at room temperature (RT) and 360 MPa at elevated temperature (ET) of 600 °C was developed based on MX (M=Nb, V, Mo; X=C, N) precipitation strengthening. A series of tensile and constant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transformation (CCT) as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal simulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was determined as 653 °C, and the granular bainite was obtained when the cooling rate was higher than 10 °C/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates (Nb, Mo, V)C with size of less than 10 nm precipitated in ferrite at ET (600 °C), which resulted in precipitation strengthening at ET.

PRECIPITATION BEHAVIOR OF NANOMETER-SIZED CARBIDES IN Nb-Mo MICROALLOYED HIGH STRENGH STEEL AND ITS STRENGTHENING MECHANISM

Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide pre*国家重点基础研究发展计划项目2015CB654803和国家高技术研究发展计划项目2015AA034302资助 收到初稿日期: 2015-09-15,收到修改稿日期: 2016-01-05 作者简介:张正延,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00482

Effect of dissolved niobium on eutectoid transformation behavior

The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied. Dissolved niobium is important in the eutectoid transformation behavior. It increases the eutectoid carbon content significantly (by ~0.0477% per 0.000 01% dissolved niobium), increases the hardenability of steel markedly, yields finer, more uniform, polygonal proeutectoid ferrite, induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal, and increases the misorientation angle of pearlite colonies from being focused near 0° to near 60°.

Effect of Mo Addition on the Precipitation Behavior of Carbide in Nb-Bearing HSLA Steel

The effect of Mo addition on the precipitation of carbides both in austenite and ferrite of Nb-bearing HSLA steel were investigated by stress relaxation and transmission electron microscopy (TEM). Experimental results showed that the addition of ~0.2 wt.% Mo into Nb-bearing steel slightly accelerated the precipitation kinetics of Nb carbides both in austenite and ferrite. The particles, (Nb, Mo)C, precipitated in Nb-Mo-bearing steel exhibit superior coarsening resistance compared to that of NbC particles in Nb-bearing steel. And the difference of carbides precipitated in anstenite and ferrite of Nb-Mo-bearing steel was discussed.

Grain Refinement and Toughening of Low Carbon Low Alloy Martensitic Steel with Yield Strength 900 MPa Grade by Ausforming

Instead of off-line quenching and tempering (QT), on-line ausforming (non-recrystallization controlled roling) and direct quenching (DQ) was employed to improve the toughness of low alloy steel with yield strength of 900 MPa Grade. Low carbon content ensured a high level of upper shelf energy, while the fine-grained martensite substructures decreased the ductile-to-brittle transition temperature and compensated the strength loss due to carbon reduction. Two mechanisms for the refinement of martensite substructure were proposed: one was the austenite grain refinement in the thickness direction, and the other was the self-accommodation of martensite variants due to austenite grain hardening. More than 200 J of Charpy V-notch impact absorbed energy at 233 K was obtained in the industrial ausformed plate, which was about the double in the traditional QT steel with the same strength grade.

A Novel Low-cost Hot Rolled High Strength Steel for an Automatic Teller Machine

A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. The low-cost C-Mn steel was microalloyed with 0.08 mass%–0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical performance, microstructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.